Slow-release nitrogen fertilizer combined with split urea application under side banding improves rice quality by coordinating carbon–nitrogen metabolism and grain filling

Side banding and slow-release nitrogen fertilizers (SRF) are effective in improving N use efficiency in rice. However, little is known about how side banding SRF combined with conventional urea applied at different growth stages regulates grain carbon (C) –nitrogen (N) metabolism and grain filling dynamics, thereby affecting rice quality. Seven treatments were established under equal N levels: no N (0N), conventional fertilization (CK), 100% urea with side banding (DU), 100% SRF with side banding (DSU), and DSU combined with 30% urea applied as basal (DSU+U), tillering (DSU+TU), or panicle fertilizer (DSU+PU). Compared with CK, SRF treatments enhanced ADP-glucose pyrophosphorylase (ADPG), soluble starch synthase (SSS), and starch branching enzyme (SBE) activities, increased total starch (5.72%–12.52%) and amylopectin accumulation (8.78%–17.97%), but reduced Granule-bound starch synthase (GBSS) and N-metabolizing enzyme activities, resulting in decreases in amylose (4.83%–6.99%) and protein content (3.51%–10.34%). DSU+PU showed the highest total starch and amylopectin and the lowest amylose, while its protein content, though lower than CK, remained higher than other SRF treatments. SRF also reduced the average grain filling rate but prolonged the active filling period, with DSU+PU extending it by 3.61 days, leading to more sufficient filling. In addition, SRF improved head rice rate and yield (by 10.04%–23.28%), reduced chalky grain rate and chalkiness, with DSU+PU showing the best performance, and increased peak viscosity, break down, and taste value (1.95%–3.52%). Comprehensive evaluation indicated that DSU+PU achieved the best performance. Collectively, DSU+PU exhibited superior coordination of C-N metabolism and grain-filling balance, achieving simultaneous optimization of yield and quality.